Field of Invention
The present invention relates generally to an optical filtering system in a sensor configured to detect an analyte within a medium within a living animal. The present invention also relates to an optical filtering system having low sensitivity to high angle of incidence light.
Discussion of the Background
A sensor may be implanted within a living animal to measure an analyte in a medium within the living animal. Examples of implantable sensors employing an analyte indicator to measure an analyte are described in U.S. Pat. No. 8,233,953 and U.S. Patent Application Publication Nos. 2013/0211213, 2013/0241745, and 2013/0324819, all of which are incorporated by reference in their entireties.
FIG. 1 illustrates a cross-sectional view of an example of an existing sensor 100. FIG. 2 illustrates a cross-sectional view of the existing sensor 100 in operation. FIG. 3 is a schematic view of the existing sensor 100. FIG. 4 illustrates various sources of light in the optical system of the optical system of the existing sensor 100. The sensor 100 includes a light source 108 that emits excitation light 129 (e.g., at an excitation wavelength of 378 nm) to an analyte indicator 106 (e.g., a polymer graft) containing indicator molecules 104 (see FIG. 3). The indicator molecules 104 have an optical characteristic that varies based on the concentration of the analyte in the medium. In particular, when excited by the excitation light 129, indicator molecules 104 that have bound the analyte emit (i.e., fluoresce) light 131 having a wavelength different than the wavelength of the excitation light 129 (e.g., the emission light 131 may have a wavelength range of about 400 nm to 500 nm with a peak emission wavelength around 435 nm) (see FIG. 4). Higher analyte levels correspond to a greater amount of emission light 131 of the indicator molecules 104 in the analyte indicator 106, and, therefore, a greater amount of photons striking a first photodetector (e.g., photodiode) 110.
The sensor 100 includes a first dichroic band pass filter 111 (thin film) that filters light incident on the first photodetector 110. The first dichroic band pass filter 111 is designed to only pass light having the wavelength of the light emitted by the indicator molecules 104 (e.g., light within the range of about 400 nm to 500 nm) so that, in theory, the first photodetector 110, which is a signal photodetector, only receives the light emitted by the indicator molecules 104.
In sensors having multiple channels (e.g., a signal channel and a reference channel) and/or multiple photodiodes, the sensor may include a dichroic band pass filter for each channel and/or photodetector. For instance, as shown in FIGS. 1-3, existing sensor 100 includes a second dichroic band pass filter 113 (thin film) that filters light incident on a second photodetector 112. The second dichroic band pass filter 113 is designed to only pass light having the wavelength of reference light so that, in theory, the second photodetector 112, which is a reference photodetector, only receives the reference light. In the existing sensor 100, the first photodetector 110 and the second photodetector 112 are arranged symmetrically on either side of the light source 108.
In the existing sensor 100, the dichroic band pass filter 111 is coated onto a glass slide 220, which is then attached to the photodetector 110, and the dichroic band pass filter 113 is coated onto a glass slide 222, which is then attached to the photodetector 112. In existing sensor 100, light (e.g., reflected excitation light 129 and fluorescent light 131 emitted by the indicator molecules 104 in the analyte indicator 106) passes through one or more glass slides 220 and 222.
The existing sensor 100 includes a sensor housing 102 (i.e., body, shell, capsule, or encasement), which may be rigid and biocompatible. The sensor housing 102 is formed from a suitable, optically transmissive polymer material (e.g., epoxy), such as, for example, acrylic polymers (e.g., polymethylmethacrylate (PMMA)). The sensor housing 102 may be any shape suitable for implantation into a living animal. The existing sensor 100 includes a substrate 116 and an encoder 118 that encodes the data before it is conveyed to an external transceiver.
In practice, the dichroic filters 111 and 112 allow the passage of light that was not intended to pass through, which may degrade the accuracy of the sensor. Accordingly, there is a need for sensors having improved accuracy and in which these problems are substantially reduced or eliminated.